The present invention relates to the general field of turbine engines. It applies in preferred but non-limiting manner to the field of aviation.
More particularly, the invention relates to a method and a device for producing a setpoint signal for use in a servo-control loop controlling a fuel metering device in a turbine engine such as a turbojet. Typically, the setpoint signal is a signal representative of a fuel mass flow rate that is to be supplied by the metering device to a fuel injection system of a combustion chamber of the turbine engine.
In known manner, the fuel metering device of a turbine engine has a valve, also referred to as a fuel metering valve (FMV), and the position of the valve is proportional to the fuel flow rate that is to be injected into the combustion chamber so long as the pressure difference across the valve is kept constant.
An estimate of the fuel mass flow rate that is to be injected into the combustion chamber is evaluated by the turbine engine computer and is supplied to the metering device in the form of a setpoint signal. One technique that is conventionally used for identifying the position of the valve on the basis of the estimated mass flow rate supplied by the computer is as follows.
On being moved, the valve obstructs a fuel flow section S through the metering device to a greater or lesser extent. The section S is also referred to as the aperture area of the metering device and it is easily expressed as a function of the position of the valve.
In known manner, this area S is proportional to the volume flow rate of the fuel. This is obtained by taking the mass flow rate setpoint signal supplied by the computer and dividing it by the density (mass per unit volume) of the fuel flowing through the metering device.
The density of the fuel is generally assumed to be constant throughout the mission of the turbine engine, and to be predetermined.
Unfortunately, that assumption does not take account of any possible change in the nature of the fuel used or any variation in the temperature of the fuel while the turbine engine is in operation. As a result it leads to inaccuracies in the evaluation of the position of the valve of the metering device.
There thus exists a need to have a technique that enables the fuel fed to a turbine engine to be metered accurately without suffering from the above-mentioned drawbacks, and in particular to produce a setpoint signal that is accurate for servo-controlling the metering device.
Document U.S. Pat. No. 5,305,597 proposes a method of producing a setpoint signal for a fuel metering device that makes use of a measurement delivered by a flow meter of the flow rate that is injected into the combustion chamber. That measurement is used for estimating a calibration signal proportional to the instantaneous density of the fuel.
The calibration signal serves to adjust the mass flow rate setpoint signal as evaluated on the basis of the position of the valve of the metering device. It is updated whenever, on the basis of a predefined criterion, it is determined that the measurement delivered by the flow meter is sufficiently accurate, i.e. typically once the measured flow rate is sufficiently large and varies little.